Magnetron amplifier



Patented Apr. 25, 1939 UNITED STATES MAGNETRON AMPLIFIER Robert Serrell,Haddonfield, N.

Radio Corporation of Delaware Application July 17,

8 Claims.

My invention relates to magnetron amplifiers. More particularly myinvention is a magnetron amplifier in which the magnetic field isautomatically compensated to maintain optimum amplification.

A magnetron is comprised of a cylindrical anode, and a small cylindricalcathode concentrically mounted within an evacuated envelope. A magneticfield is applied so that its lines of force surround the cathode and aresubstantially parallel thereto. If the cathode is heated and voltageapplied between cathode and anode, electrons emitted from the cathodeand under the influence of the magnetic field will follow curved pathsto or towards the anode.

The curvature of the path depends upon the physical arrangement of thecathode and anode, the applied voltage, cathode emission and density ofthe applied magnetic field. If the'magnetron 20 elements are suitablyproportioned, the number of electrons reaching the anode can be variedfrom a maximum to zero by simply increasing the strength of the magneticfield.

One of the objects of my invention is to provide 25 means for utilizingthe relation between the anode current and the applied magnetic field toeffect uniform amplification over a wide frequency band in a magnetron.

Another object is to provide means for maintaining a linear relationshipbetween the applied field and anode current in a magnetron amplifier.

A further object is to provide, in a magnetron amplifier, an inputcircuit which varies the magnetic field and a resistive anode circuitwhich includes means, controlled by the varying anode voltage, formaintaining a predetermined relationship, over a frequency range of amillion cycles and upward, between input and output currents in saidamplifier.

My invention may be best understood by reference to the accompanyingdrawing in which Fig. 1 r is a schematic circuit diagram of a magnetronamplifier embodying one application of my invention, and

Fig. 2 is a set of characteristic curves showing the relation betweenthe anode current and applied magnetic field of a magnetron.

In Fig. 1 an input circuit is represented as a solenoid I. The solenoidmay be coaxially posi- 0 tioned with respect to the electrodes of amagnetron 3. The magnetron 3 is comprised of an evacuated envelope 5, acathode 1, a cylindrical anode 9 coaxially arranged with respect to thecathode, and a main magnetic field supplied by a suitably energizedsolenoid it which is shown J., assignor to America, a corporationof1936, Serial No. 91,204

partly in section. The cathode is energized by a battery I I which isgrounded. The anode 9 is connected through an output circuit resistor 33to the positive terminal of an anode battery is. The negative terminalof this battery is grounded. An output circuit I1 is connected betweenthe anode 9 and ground I9. The resistive output circuit may be madesubstantially independent of frequency over a range of a million cyclesand upward. That is the output circuit is substan- 10 tially independentof reactive components, or has an impedance which is substantiallyindependent of the frequency of the applied currents.

A compensating magnetic field for the magnetron is supplied by asolenoid 25. This solenoid may surround the magnetron 3 and be coaxiallyand symmetrically arranged with respect to the input solenoid l and mainsolenoid iii. The sole noid 2| is connected between the anode 23 ofthermionic tube 25 and the positive terminal of an anode battery 21. Thenegative terminal of the battery 2'! is connected to the anode Q or themagnetron 3.

The suppressor grid 29 of tube is connected to the cathode 32 which isenergized by a battery 25 33. The screen grid 35 is connected to thepositive terminal or a battery 31. The negative terminal of the battery31 is joined to the positive terminalof the magnetron anode battery 55.The control grid 39 of thermionic tube 25 is biased by 3 an appropriateconnection located intermediate the terminals of the anode battery i5,or the control grid may be suitably biased by'a separate battery.

The relation between anode current in milli- 5 amperes and the magneticfield in gauss of a conventional magnetron is represented by the graph Aof Fig. 2. This characteristic is based upon an anode circuit ofnegligible resistance and a constant anode potential. If a resistance is4 included in the anode circuit of the magnetron, the variation of anodecurrent with variations of the applied field will alter thecharacteristic curve. Because the anode potential is no longer constantbut varies, a magnetic field of varying strength is required to maintainthe characteristic A. That is, as the anode potential varies, otherfactors remaining constant, the characteristic curve changes, asindicated by the graphs B and C of Fig.2.

Such Variation in the characteristic curve would be undesirable in anamplifier because a less abrupt slope or shorter slope would decreasethe amplification factor and a curving or non-uniform characteristicwould introduce distortion. 55

If the characteristic A is to be maintained and including cathode andanode electrodes, a source distortion avoided, the main magnetic fieldmust be compensated.

I have found that suitable compensation may be obtained by a thermionictube and circuit, such as the pentode connection illustrated in Fig. 1.As is shown, the bias on the grid 39 is fixed; under these circumstancesthe plate or anode current of the pentode is proportional to the squareroot of the anode voltage. The compensating coil 2| therefor hasincrements of current flowing through it which are proportional to thesquare root of the signal voltage across the anode resistor l3. Theseincrements of current, which are proportional to the output signalcurrents, establish a compensating magnetic field which maintains thedesired relationship between input and output currents.

It should be understood that the main magnetic field is initiallyadjusted so that the operating point of the magnetron amplifier issubstantially in the center of the sloping portion of the characteristiccurve A. The input currents alter the magnetic field and anode currentbut the compensating pentode circuit automatically varies the magneticfield, and thereby shifts the operating point with the signal to obtainthe maximum linear variations of the magnetron anode current.

Thus I have described a magnetron amplifier in which the characteristiccurve is utilized to its full advantage by automatically shifting theoperating point with varying signal output currents. The resistiveoutput circuitinsures an output which is substantially independent offrequency and makes amplification over a wide range of frequenciespractical. Although I have shown a plurality of solenoids, it should beunderstood that a single solenoid winding with suitable taps, or severalsolenoids with or without magnetic cores may be used. Likewise anycompensating circuit may be used provided it substantially follows thelaw that the current through the compensating solenoid is proportionalto the square root of the voltage changes across the magnetron anodecircuit resistor.

I claim as my invention:

1.- An amplifier comprising a magnetron having cathode and anodeelectrodes, means for creatin a magnetic field for said magnetron, meansfor varying said field by a second magnetic field established bycurrents to be amplified, a source of anode energizing potential, anoutput circuit resistance traversed by the anode currents from saidmagnetron, whereby variations in potential are produced across saidoutput resistance which are proportional to variations in anode currentand means including a thermionic tube responsive to said voltagevariations for compensating for changes in the characteristic curve ofsaid magnetron caused by potential variations of said anode.

2. A magnetron amplifier comprising an evacuated envelope containingcathode and anode electrodes, means for establishing a magnetic fieldwhose lines of force surround and are substantially parallel to saidcathode, means for varying said field with input currents to beamplified, means for producing voltage variations proportional to anodecurrent variations comprising an output resistance connected to saidanode, and means including a thermionic tube responsive to said voltagevariations for compensating for the effects of said voltage variationson the amplification characteristic of said tube.

3. An amplifier comprising a magnetron tube of anode power and aresistive output element serially connected between said source of anodepower and said anode electrode, means for establishing a magnetic fieldfor effecting a curved path of electrons moving from said cathode tosaid anode electrodes, means for varying said magnetic field by a secondmagnetic field created by the currents to be amplified, and meansincluding a thermionic tube responsive to the voltage across saidresistive output element for correcting the effects of varying anodepotentials caused by potential variations across said resistive outputelement.

4. An amplifier comprising a magnetron, means for creating a magneticfield for said magnetron, means for varying said field by a secondmagnetic field established by currents to be amplified, a resistiveoutput element traversed by amplified output currents, and meansincluding a thermionic tube responsive to voltage changes across saidoutput element for establishing a compensating magnetic field bycurrents which vary as the square root of the voltage across saidresistive output element.

5. An amplifier comprising a magnetron, means for creating a magneticfield for said magnetron, means for impressing input currents on saidmagnetron, a resistive output element traversed by amplified outputcurrents, and means including a thermionic tube responsive to voltagechanges across said output element for creating a magnetic field bycurrents in said tube which substantially vary as the square root of thevoltage across said resistive output element.

6. An amplifier comprising a magnetron having a cathode and anodeelectrodes, means for creating a magnetic field for said magnetron,means for impressing input currents on said magnetron, a resistiveoutput element, traversed by amplified anode currents, and meansincluding a thermionic tube responsive to voltage changes across saidoutput element for creating a magnetic field for compensating forchanges in the characteristic curve of said magnetron caused bypotential changes across said resistive output element.

7. An amplifier comprising a magnetron, means for creating a magneticfield for said magnetron, means for impressing input currents on saidmagnetron, a resistive output element traversed by amplified outputcurrents, a thermionic pentode having fixedly biased suppressor, screen,and control grids, a solenoid connected to the anode electrode of saidpentode and serially including the resistive output element of saidmagnetron whereby a compensating magnetic field is created by currentsfiowing in said solenoid, and means for applying said compensating fieldto said magnetron.

8. An amplifier comprising a magnetron, means for creating a magneticfield for said magnetron, means for impressing input currents on saidmagnetron, a resistive output element traversed by amplified outputcurrents, a thermionic pentode having fixedly biased suppressor, screen,and control grids, a solenoid connected to the anode elec trode of saidpentode and serially including the resistive output element of saidmagnetron whereby a compensating magnetic field is created by saidsolenoid, and means for applying said compensating field to saidmagnetron to vary the total magnetron field by current increments whichvary as the square root of the voltage changes in said resistive outputelement.

ROBERT SERRELL.

